British Airways has been accused of greed as it continues to deal with the fallout from a global IT crash which unions say could have been avoided if jobs had not been outsourced.
A third day of disruption lies ahead as both Heathrow and Gatwick have warned Bank Holiday travellers they should check the status of their flights before travelling to the airports where scenes of chaos unfolded over the weekend.
The GMB said the disruption could have been prevented if the beleaguered airline had not cut "hundreds of dedicated and loyal" IT staff and contracted the work to India in 2016.
Mick Rix, national officer for aviation at the union, said at the weekend: "We can only feel genuinely sorry for the tens of thousands of passengers who are stranded at airports and face having their travel plans and holidays ruined.
"This could have all been avoided. In 2016 BA made hundreds of dedicated and loyal IT staff redundant and outsourced the work to India.
"BA have made substantial profits for a number of years, and many viewed the company's actions as just plain greedy."
The move in 2016 sparked protests and outrage from members of the union, and at the time a BA spokesman said: "A contract has been signed with TCS to be the supplier of some IT activities in British Airways, and British Airways has been in consultation with those IT staff affected, about 200."
BA has said it will run a full schedule at Gatwick on Monday and it intends to operate a full long-haul schedule and a "high proportion" of its short-haul programme at Heathrow.
The airline said it was continuing to make "good progress" in recovering from the worldwide IT glitch that grounded scores of planes, leaving thousands of passengers stranded.
An airline spokeswoman said: "We operated a full schedule at Gatwick on Sunday.
"At Heathrow, we operated virtually all our scheduled long-haul flights, though the knock-on effects of Saturday's disruption resulted in a reduced short-haul programme.
"As our IT systems move closer to full operational capacity, we will again run a full schedule at Gatwick on Monday and intend to operate a full long-haul schedule and a high proportion of our short-haul programme at Heathrow.
"We apologise again to customers for the frustration and inconvenience they are experiencing and thank them for their continued patience."
Travellers spent the night sleeping on yoga mats spread on terminal floors on Saturday after BA cancelled all flights leaving the London hubs, while disruption continued into Sunday with dozens more services from Heathrow axed.
The IT outage had a knock-on effect on BA services around the world, while passengers who did get moving on the limited number of flights to take off from the UK reported arriving at their destinations without their luggage.
The disruption also hit transport systems on the ground, with hundreds of travellers flooding London's Kings Cross station in hope of boarding a train north instead.
Experts predict the knock-on effect could continue for several days and BA is facing huge compensation costs, with reports suggesting the bill could top £100 million.
The airline blamed the computer blackout on a "power supply issue" and said there was no evidence of it resulting from a cyber attack.
Issuing an apology on Sunday, the airline's chief executive, Alex Cruz, admitted it had been "a horrible time for our customers".
On Monday a Heathrow Airport spokeswoman said: "Following a worldwide IT system issue, there continues to be some disruption to British Airways flights from Heathrow.
"All British Airways passengers due to fly should check the status of their flight before travelling to the airport, via Heathrow.com, ba.com and British Airways' Twitter account.
"All passengers whose flights have been cancelled should not travel to the airport unless they have already rebooked onto another flight. Passengers looking to re-book flights should go to ba.com"
Meanwhile Gatwick Airport tweeted: "Today, Monday 29th May, British Airways are planning to operate a near normal schedule at Gatwick, following the British Airways IT system failure.
"We continue to advise customers travelling with British Airways over the Bank Holiday Weekend to check the status of their flight with British Airways before travelling to the airport.
"Customers should not travel to the airport unless they have already rebooked onto another flight."
A Heathrow spokeswoman said: "Following a worldwide IT system issue, there continues to be some disruption to British Airways flights from Heathrow.
"All British Airways passengers due to fly should check the status of their flight before travelling to the airport, via Heathrow.com, BA.com and British Airways' Twitter account.
"All passengers whose flights have been cancelled should not travel to the airport unless they have already rebooked onto another flight. Passengers looking to re-book flights should go to BA.com
"We will continue to do all we can to help British Airways passengers affected. We have mobilized additional Heathrow colleagues to assist passengers at the terminals and give out free water and snacks. We are working hard with British Airways to reunite passengers with their bags as soon as possible.
"We are very sorry to any passengers whose journeys have been affected."
Travellers were facing only a handful of cancellations at Heathrow on the third day of disruption since the IT glitch.
The atmosphere inside Terminal Five appeared to be getting back to normal, with much of the backlog of stranded passengers caused by Saturday's outage seeming to have eased.
Display boards showed some flights cancelled, including planes to Copenhagen and Amsterdam.
The other cancellations were domestic flights to Glasgow, Aberdeen, Manchester, Leeds Bradford and Dublin.
While a number of weary-looking passengers filled the benches by departures, for the majority check-in seemed to be going smoothly.
Extra staff wearing purple were dotted around the terminal to assist passengers.